A current transducer of the Rogowski type incorporates a device which is commonly known as a Rogowski Coil. It is widely used as a device for measuring alternating current (AC) or high frequency current pulses. This type of coil has many advantages over other types of current sensors. Known Rogowski coils can be constructed by applying an electrically conductive wire on a non-magnetic and non-conductive carrier. The carrier can be a plastic or glass or ceramic based structure and forms a closed or almost closed loop such that a kind of helical coil wire is formed. The lead from one end of the coil can return through the center of the coil or close to center of the coil to the other end, so that both terminals can be at the same end of the coil and so that the helical coil itself does not form a closed loop.
The Rogowski coil belongs to the category of air-core coils because the carrier of the coil is non-magnetic, e.g., its magnetic susceptibility is significantly smaller than 1. The carrier can be rigid or flexible and its shape can be a ring, torus, or other.
When placed around a primary conductor carrying an electrical current, the Rogowski coil generates a voltage proportional to the derivative of the current according to Ampere's law. The voltage is also proportional to the number of turns per unit length and to the area of the turns. The area of one turn is approximately equal to the cross-sectional area of the coil carrier. Because the voltage induced in the Rogowski coil is proportional to the rate of change of the current in the primary conductor, the output of the coil can be connected to an electronic device, here called transducer electronics or Intelligent Electronic Device (IED), where the signal is integrated and further processed in order to provide an accurate signal that is proportional to the current.
The Rogowski coil has many advantages over other types of current measuring devices, the most notable being the excellent linearity due to its non-magnetic core, which is not prone to saturation effects. Thus, the Rogowski coil is highly linear even when subjected to large currents, such as those used in electric power transmission, welding, or pulsed power applications. Furthermore, because a Rogowski coil has an air core rather than a magnetic core, it has a low inductance and can respond to fast changing currents. A properly formed Rogowski coil, with equally spaced windings, is largely immune to electromagnetic interference. In comparison to known ferromagnetic core based current transducers, a Rogowski coil current transducer (RCCT) exhibits a higher dynamic range, lower weight and size, as well as lower production cost.
However, known Rogowski coil current transducers provide moderate accuracy as compared to high known ferromagnetic core based current transducers, particularly for metering applications. One reason for this is the unknown changes of sensitivity S of Rogowski coils when the environmental conditions can be changing, such as temperature, mechanical constraints, humidity, aging etc. Another reason is the unknown changes of gain in electronic amplifiers being part of the transducer electronics when the environmental conditions can be changing, such as temperature, mechanical constraints, humidity, aging etc.
Because these non-desirable changes of coil sensitivity and of amplifier gain can be currently not taken into account by the electronic signal processing in the IED, such an alteration in the sensitivity and in the gain introduces an error on the measurement. Such a limitation impedes reaching high accuracy with a combination of a Rogowski coil current transducer.
One solution to compensate for sensitivity changes of known Rogowski coil current transducers includes measuring the temperature with a temperature sensor placed closed to the Rogowski coil. The temperature is then used to compensate the sensitivity according to each Rogowski sensitivity temperature profile. During the characterization (e.g., calibration) of the coil performed at the end of production, the Rogowski coil current transducers sensitivity at ambient temperature as well as its temperature dependency can be measured. The coefficients, which give the polynomial correction to apply to the signal in the transducer electronics, can be stored in an EEPROM placed in the sensor casing. This solution allows temperature effect compensation, but specifies additional production effort, such as calibration and temperature characterization of each Rogowski coil current transducer. Furthermore, it does not allow other compensations such as of mechanical, humidity and aging effects. Indeed, the correction coefficients cannot be updated once the sensor is delivered to the customer. In fact, in order to compensate for aging, the known approach can call for a maintenance effort and an interruption of the rated current measurement on the customer plant. The known Rogowski coil current transducer should be extracted from the plant and recalibrated periodically with the same procedure as the initial calibration in the factory.
PCT/EP2011/001941 and PCT/EP2011/058291 propose readout electronic arrangements that compensate themselves (e.g., self-compensate) their own gain drift without interrupting the measurement via an online capability. However, these readout electronics do not allow for the compensation of any RCCT sensitivity variation simultaneously with the measurement of the rated current.